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A biology/evolutionary quesiton that I should have been able to answer...

Posted on: October 29, 2008 - 11:22pm

holmcross

Posts: 2

Joined: 2008-10-29

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A biology/evolutionary quesiton that I should have been able to answer...

This is a question to the biologists out there. I’ve taken a few semesters in biology at college, but I found myself unable to give an answer I was satisfied with. I’m a chemist, not a biologist.

A little bit about me to flesh out the context... I used to be a hard-core conservative theist that believed there was overwhelming evidence to believe in God, and also to believe in a literal translation of the bible. Long story short, I found out that all my reasoning was circular and based on information which was flat out wrong. I’m still very good friends with many of the people from my old church and I argue with them from time to time.

My stance was that “I believe in religion because above everything else, that’s where the evidence leads. I’m a slave to that evidence”. It turned to “I don’t believe in God or the supernatural because I haven’t seen any real evidence to suggest the existence of such a thing” (Quite the change ).

But anyway, I was arguing with a friend of mine, and he claims that there is a slow and constant breakdown of the DNA of humans over time, and that we are a dying race. He goes on to say that this is a result of the perfect genome of adam and eve (I should have nailed him and had him define perfect) and that this is evidence to suggest a near-perfect original genome and a slow slide downhill.

To sum it up, he says that we are ‘copies’ of ‘copies’ of ‘copies’ of an original, and that with each generation, we slowly degrade. That’s his belief.

I’m rusty on my cellular meiosis and mitosis, but my answer to ‘DNA breaking down” (damn, I should have really nailed him for that terminology… haha) was that meiosis is a separate process then mitosis… and its mitosis that he is obviously referencing when he talks about genetic breakdown (aging). I still wasn’t sure how to answer his question though, because yes, over time our DNA does become damaged (beyond what our enzymes can normally repair) but that dosn't mean that we are 'breaking down'. I assume that he believes judgement day is either on the day that we lose the ability to pro-create, or before it.

As for this discrediting evolution, I explained to him that evolution is the change in a population’s allelic distribution over time. More DNA does not mean a ‘better’ organism: the arrangement of the bases within DNA is that causes mutation/physical traits. So suggesting that more genetic material is better (iirc the vast majority of genetic information is condensed around the histone protiens is junk and not expressed) is not how evolution functions. So the conception that we are losing bits of data over time.

I also told him that genetic diversity, the fact that two organisms both donate to the offspring, helps to add new genetic information and pickup where one partner may have left off. I was weak on providing the mechanics to explain this (this is first year bio stuff, ffs and I stuffed it).

Isn’t it annoying that the non-believer does all the scientific legwork? It’s essential to force the person making the claim (as he was, claiming that we are breaking down) to supply their own body of evidence, but I was sure I could explain to him quickly why that’s a common misconception. Now it has grown into a nagging question that’s bothering me. I know that when I give him a better answer he’ll just shrug it off and move to the next little nit-picky issue he has. But I want to know for my own reasons.

Also, it’s important to show the believer that my stance is rational and can be defended through logical and scientific means. After that I can put the ball in his court.

So yeah, thanks for the help if anyone can field this. I skimming my old notes and bio text, but couldn’t find something which addressed this idea specifically. Thanks for reading the wall of text.

he claims that there is a slow and constant breakdown of the DNA of humans over time

This is flat out false. It was a puzzle contemplated by late 18th century biologists, which was refuted by the formalization of the Second Law of Thermodynamics. Prior to the formalization of thermodynamics, it was thought that biological organisms died because they broke down over time, by wear and tear. But when the laws of thermodynamics were formalized, it was shown that any open system can continue to maintain a low entropy as long as the sum total decrease in entropy in the local system resulting from the injection of free energy is less than the total increase in entropy in the surroundings. In principle, therefore, a biological organism can continue to maintain itself as long as this condition is met. Certain cell ines, like those of the yeast, proliferate indefinitely since they control their telomere length. The fidelity of DNA replication does not decrease in the somatic line of the organism over time. For example, some cells, like the budding yeast, are basically immortal. In principle, DNA replication can continue indefinitely, in fact, via the germ line it does. Obviously the passing of DNA is not a perfect duplicate of the parent copy, and there is much room for mutation, recombination and homology as evidenced by the fact that the biological germ line in every single human on this planet can be traced back to unicellular bacterial organisms.

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and that we are a dying race

This is false. Genomes do not deteriorate over time. Quite the opposite. Every undergraduate of genetics and molecular biology learns that unless under natural selection, stretches of nucleotides will be destroyed over time by the process of random mutation. Sequences not under selection are called non-conserved sequences. Seriously, ask any geneticist, any molecular biologist, any proteomicist, any evolutionary biology, or any undergraduate in any of these areas and they will immediately reply with the same answer: "high fidelity of a genome can only be maintained under natural selection".

In modern phylogenetics, rates of evolution are quantified in terms of base pair substitutions. According to the principle called the molecular clock, the differences in any two homologous nucleotide sequences is directly proportional to the time since those two sequences diverged. Also, according to the hypothesis, the rate at which base pair substitution occurs in any nucleotide sequence is determined by how conserved the sequence is. The more conserved the sequence is, the less substitution there is. For example, in eukaryotes, a protein called Histone H2 is extremely highly conserved because it is extremely important to all Eukaryotic organisms and virtually all point mutations in the protein are fatal. As such, the difference between the human H2 and, say, the histone H2 in cows, is 2 nucleotides per 1000. This is tiny. The reason that some sequences will mutate at faster rates than others is because in certain more conserved sequences, mutations are more deleterious, therefore eliminated by natural selection. On the other hand, a sequence which has no conservation whatseover will mutate solely on the basis of purely random frequency. This principle is very important. Sequences which have no use or conservation will simply mutate out of existence. There is no such thing as "dormancy" in evolutionary biology. If selection pressure is relaxed, it (the sequence) will be destroyed.

Here's an example of the predictive usefulness of the molecular clock

The other thing to stress is that evolutionary rates are highly constrained. Populations need to be able to adapt to the environment by means of changes in the lineage caused by the propagation of advantageous variations in individuals due to genetic makeup, otherwise they will die out. On the other hand, if the rate of mutation is too high, the deleterious mutations will be overwhelming and the organism will be rapidly destroyed, as high levels of cancers and premature deaths result from an excess of deleterious mutations. As such, changes in evolutionary rates need to fall within this balance.

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To sum it up, he says that we are ‘copies’ of ‘copies’ of ‘copies’ of an original, and that with each generation, we slowly degrade. That’s his belief.

It is quite easy to nail this to the wall. The genomes of individual organisms (which should not be confused with the conservation of genetic sequences over generations) can break down over time. The reason for this is not known. It may be due to the shortening of telomeres, which withdraw their chromosomes from the cell cycle once there is no longer an Okazaki fragment at the end of the chromosome in question, as the ends will be broken. According to this hypothesis, called replicative cell senescence, mammals are born with a set number of telomeres. However, since telomerase is active in the germ cells, the telomeres do not shorten over generations, unless it is deliberately switched off via experimentation. The other main hypothesis usually refers to free radical damage over time as being primarily responsible for the accumulation of errors in the genomes of individual organisms. However, this is ultimately irrelevant from the standpoint of the propogation of genes, since these processes do not become noticeable until long after the organism in question is sufficiently mature to pass on its genetic material. That's the key point.

"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.

Thanks for your response, DG, I couldn't have asked for a better answer . Sorry for taking so long to get back to you.

I'm trying to figure out a way to word it to him, since he has no real education in molecular cell bio or chemistry for that matter. Honestly, I think it may be in line to just call him on his assumptions. I think this is where he is comming up with this idea:

A copy of a copy will be less perfect then the original, therefore, over time our genome is breaking down. I should remind him that our genes are not the same as recording an analog signal from a cassette tape onto a new tape!

If he continues to stress the fact, I'll simply ask for his sources and for him to explain his theory. Without him having even a basic grasp of biology or chemistry, it would be very difficult to try to explain this to him in the terms that you did. I even had to read it through a few times, and I'm a chem major for freaking sakes!